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2.0 DIODES

2.1 Understand the characteristic of diode and diode as a semiconductor device

schematic diagram and physical structure of diode.

Diodes are used to allow current to flow in one direction while blocking current flow in the oppositedirection.

Forward biased and Reverse biased.

Diode must be connected the correct way around. Small signal diode can be damaged by heat whensoldering unless when using a germanium diode.

The Biased P-N Junction

The p-n junction is considered biased when an external voltage is applied. There are two types ofbiasing: Forward bias and Reverse bias.

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Forward Bias:

Vapplied > 0

In forward bias the depletion region shrinks slightly in width. With this shrinking the energy requiredfor charge carriers to cross the depletion region decreases exponentially. Therefore, as the applied

voltage increases, current starts to flow across the junction. The barrier potential of the diode is thevoltage at which appreciable current starts to flow through the diode. The barrier potential varies fordifferent materials.

Electrons in the p-type material, near the positive terminal of the supply, break their electron pairbonds and enter the supply, thereby producing new holes. Also electrons from the negative terminalof the supply enter the n-type material and migrate towards the junction.

Free electrons from the n-type then flow across the junction and move into the holes which havemigrated from the positive terminal. This current flow will continue as long as the external supply isconnected and is called forward current flow.

Reverse Bias:

Vapplied < 0

Under reverse bias the depletion region widens. This causes the electric field produced by the ions tocancel out the applied reverse bias voltage. A small leakage current, Is (saturation current) flowsunder reverse bias conditions. This saturation current is made up of electron-hole pairs beingproduced in the depletion region. Saturation current is sometimes referred to as scale currentbecause of its relationship to junction temperature.

When the polarity of the supply is reversed, the potential within the device is effectively reinforced and

the depletion layer becomes wider. This is because the free electrons in the n-type are attractedtowards the positive terminal, away from the junction, while the electrons from the negative terminal of

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the supply enter the p-type and migrate towards the junction. Current flow is extremely small and iscalled reverse current. Note that this current is produced by minority carriers and the device is said tobe reverse biased.

I V characteristic curve for silicon diode.

Diode Characteristics: I-V characteristic

In forward bias operation, the diode will not conduct significant current until the voltage reachesabout 0.7V. After that point large increases in current cause little change in voltage.

In reverse bias operation, the diode will not conduct significant current until some breakdownthreshold voltage which is typically quite large (e.g. 200V). This voltage must be somewhat greaterthan the peak input voltage (PIV) rating of the diode. (Device dependent)

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Threshold Voltage (Knee Voltage)

Definition : the minimum voltage for diode to ON (0.7 V for silicon and 0.3 V for Germanium)

Forward Current

Definition : current that flows under the application of a forward voltage

Reverse Current

Definition : small value of direct current that flows when a semiconductor diode has reverse bias (inmicroampere scale)

Breakdown Voltage

Definition : the minimum reverse voltage to make the diode conduct in reverse

Burning Level

Definition : the voltage applied exceed the maximum diode voltage and may cause the diode to burnand damage

2.2 Diode applications as rectifier.

A rectifier is an electrical device that converts alternating current (AC), which periodically reversesdirection, to direct current (DC), which flows in only one direction. The process is known asrectification. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-arcvalves, solid-state diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches.Historically, even synchronous electromechanical switches and motors have been used. Early radioreceivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (leadsulphide) to serve as a point-contact rectifier or "crystal detector".

Rectifiers have many uses, but are often found serving as components of DC power supplies and

high-voltage direct current power transmission systems. Rectification may serve in roles other than to

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generate direct current for use as a source of power. As noted, detectors of radio signals serve asrectifiers. In gas heating systems flame rectification is used to detect presence of flame.

The simple process of rectification produces a type of DC characterized by pulsating voltages and

currents (although still unidirectional). Depending upon the type of end-use, this type of DC currentmay then be further modified into the type of relatively constant voltage DC characteristicallyproduced by such sources as batteries and solar cells.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

a) Half-wave rectifier

In a half wave rectifier only one half cycle of ac voltage is taking. The circuit is given. Here only onediode is using. During the positive half cycle of ac voltage the diode conducts; so current flowsthrough load. During the negative half cycle, the diode is reverse biased .So no current flows throughthe diode. This type of rectification needs only one diode. But the efficiency is not as good as that offull wave rectifier.

When a single rectifier unit is placed in series with the load across an ac supply, it converts alternatingvoltage into uni-directional pulsating voltage, using one half cycles of the applied voltage, the otherhalf cycles beingsuppressed because it conducts only in one direction. Unless there is an inductanceor battery in the circuit, the current will be zero, therefore, for half the time. This is called half-waverectification. As already discussed, diode is an electronic device consisting of two elements known ascathode and anode. Since in a diode electrons can flow in one direction only i.e. from cathode toanode so the diode provides the unilateral conduction necessary for rectification. This is true fordiodes of all types-vacuum, gas-filled, crystal or semiconductor, metallic (copper oxide and selenium

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types) diodes. Semiconductor diodes, because of their inherent advantages are usually used as arectifying device. However, for very high voltages, vacuum diodes may be employed.

The half-wave rectifier circuit using a semiconductor diode with a load resistance RL but no smoothing

filter is given in figure. The diode is connected in series with the secondary of the transformer and theload resistance RL, the primary of the transformer is being connected to the ac supply mains.

Working of a Half wave rectifier:

The ac voltage across the secondary winding changes polarities after every half cycle. During thepositive half-cycles of the input ac voltage i.e. when upper end of the secondary winding is positivew.r.t. its lower end, the diode is forward biased and therefore conducts current. If the forwardresistance of the diode is assumed to be zero (in practice, however, a small resistance exists) theinput voltage during the positive half-cycles is directly applied to the load resistance RL, making its

upper end positive w.r.t. its lower end. The waveforms of the output current and output voltage are ofthe same shape as that of the input ac voltage.

During the negative half cycles of the input ac voltage i.e. when the lower end of the secondarywinding is positive w.r.t. its upper end, the diode is reverse biased and so does not conduct. Thusduring the negative half cycles of the input ac voltage the current through and voltage across the loadremains zero if the reverse current, being very small in magnitude, is neglected. Thus for the negativehalf cycles no power is delivered to the load.

Thus the output voltage developed across load resistance RL (VL) is a series of positive half cycles ofalternating voltage, with intervening very small constant negative voltage levels, It is obvious from thefigure that the output is not a steady dc, but only a pulsating dc wave. Since only half-cycles of theinput wave are used, it is called a half-wave rectifier.

The type of supply available from a half-wave rectifier is not satisfactory for general power supply.This type of supply can be satisfactory for some particular purposes such as battery charging.

In half wave rectification, either the positive or negative half of the AC wave is passed, while the otherhalf is blocked. Because only one half of the input waveform reaches the output, it is very inefficient ifused for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase

supply, or with three diodes in a three-phase supply. Half wave rectifiers yield a unidirectional butpulsating direct current.

The output DC voltage of a half wave rectifier can be calculated with the following two ideal equations:

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b) Full-wave rectifier

In such a rectifier, the ac input is applied through a transformer, the anodes of the two diodes D 1

and D2 (having similar characteristics) are connected to the opposite ends of the centre tappedsecondary winding and two cathodes are connected to each other and are connected alsothrough the load resistance RL and back to the centre of the transformer, as shown above.

When the top of the transformer secondary winding is positive, say during the first half-cycle ofthe supply, the anode of diode D1 is positive w.r.t. cathode, and anode of diode D2 is negativew.r.t. cathode. Thus only diode D1 conducts, being forward biased and current flows from cathodeto anode of diode D1 through load resistance RL and top half the transformer secondary makingcathode end of load resistance RL positive. During the second half-cycle of the input voltage thepolarity is reversed, making the bottom of the secondary winding positive w.r.t. centre tap andthus diode D2 is forward biased and diode D1 is reverse biased. Consequently during this half-cycle of the input only the diode D2 conducts and current flows through the load resistance RL and

bottom of the transformer secondary make the cathode end of the load resistance R L positive.Thus the direction of flow of current through the load resistance RL remains the same during bothhalves of the input/supply voltage. Thus the circuit showed acts as a full-wave rectifier.

The half-wave rectifier is very simple and does work, it is not very efficient. It only uses half of theincoming ac cycle, and wastes all of the energy available in the other half. For greater efficiency,we would like to be able to utilize both halves of the incoming ac. One way to accomplish this is todouble the size of the secondary winding and provide a connection to its centre. Then we can use

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two separate half-wave rectifiers on alternate half-cycles, to provide full-wave rectification. Thecircuit is shown to the right.

Because both half-cycles are being used, the DC component of the output waveform is now2Vp/ = 0.6366vp, where Vp is the peak voltage output from half the transformer secondary

winding, because only half is being used at a time.

This rectifier configuration, like the half-wave rectifier, calls for one of the transformer's secondaryleads to be grounded. In this case, however, it is the center connection, generally known as thecenter tap on the secondary winding.

The full-wave rectifier can still be configured for a negative output voltage, rather than positive. Inaddition, as shown to the right, it is quite possible to use two full-wave rectifiers to get outputs ofboth polarities at the same time.

The full-wave rectifier passes both halves of the ac cycle to either a positive or negative output.This makes more energy available to the output, without large intervals when no energy isprovided at all. Therefore, the full-wave rectifier is more efficient than the half-wave rectifier. Atthe same time, however, a full-wave rectifier providing only a single output polarity does require asecondary winding that is twice as big as the half-wave rectifier's secondary, because only half ofthe secondary winding is providing power on any one half-cycle of the incoming ac.

Actually, it is not all that bad, because the use of both half-cycles means that the current drain onthe transformer winding need not be as heavy. With power being provided on both half-cycles,

one half-cycle does not have to provide enough power to carry the load past an unused half-cycle. Nevertheless, there are some occasions when we would like to be able to use the entiretransformer winding at all times, and still get full-wave rectification with a single output polarity.

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The average androot-mean-square output voltages of an ideal single phase full-wave rectifiercan be calculated as:

c) Bridge rectifier

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Full wave rectifier circuit consist of 2 half wave rectifier circuits and connected to ensure

only one diode operates at one cycle. Centre-tap transformer is used to ensure both AC

voltage phases are opposition to each other.

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When the upper end of the transformer secondary winding is positive, say during first half-cyclesof the input supply, diodes D1 and D3 are forward biased and current flows through arm AB,enters the load at positive terminal, leaves the load at negative terminal, and returns back flowingthrough arm DC. During this half of each input cycle, the diodes D2 and D4 are reverse biasedand so the current is not allowed to f low in arms AD and BC. The flow of current is indicated bysolid arrows in the figure. In the second half of the input cycle the lower end of ac supplybecomes positive, diodes D2 and D4 become forward biased and current flows through arm CB,enters the load at the positive terminal, leaves the load at negative terminal and returns backflowing through arm DA. Flow of current has been shown by dotted arrows in the figure. Thus thedirection of flow of current through the load resistance RL remains the same during both halfcycles of the input supply voltage.

With the availability of low-cost, highly reliable and small-sized silicon diodes bridge rectifier isbecoming more and more popular in comparison to centre-tap rectifier. It has many advantagesover a centre-tap rectifier, as given below.

1. No centre tap is required in the transformer secondary so in case of a bridge rectifier thetransformer required is simpler. If stepping up or stepping down of voltage is not required,transformer can be eliminated even.

2. The PIV is one half that of centre-tap rectifier. Hence bridge rectifier is highly suited for highvoltage applications.

3. Transformer utilisation factor, in case of a bridge rectifier, is higher than that of a centre-taprectifier.

4. For a given power output, power transformer of smaller size can be used in case of the bridgerectifier because current in both (primary and secondary) windings of the supply transformer flowfor the entire ac cycle.

The main drawback of a bridge rectifier is that it needs four diodes, two of which conduct inalternate half-cycles. Because of this the total voltage drop in diodes becomes double of that incase of a centre-tap rectifier. Another drawback of bridge rectifier is that the load resistor RL andthe supply source have no common point which may be earthed.

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2.3 Diode Clipper and Clamper circuit.

Clipper

There are a variety of diode networks called clippers that have the ability to clip off a portion ofthe input signal without distorting the remaining part of the alternating waveform. The half waverectifier is an example of the simplest form of diode clipper one resistor and diode. Dependingon the orientation of the diode, the positive or negative region of the input signal is clipped off.

There are two general categories of clippers: series and parallel. The series configuration is

defined as one where the diode is in series with load, while the parallel variety has the diode in abranch parallel to the load.

Clampers

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Clampers have the ability to clamp a signal to a different DC level. The network must have a

capacitor, a diode and a resistive element.

clamperis an electronic circuit that prevents a signal from exceeding a certain defined magnitude

by shifting its DC value. The clamper does not restrict the peak-to-peak excursion of the signal,but moves it up or down by a fixed value. A diode clamp (a simple, common type) relies on a

diode, which conducts electric current in only one direction; resistors and capacitors in the circuit

are used to maintain an altered dc level at the clamper output.

The clampingnetwork is one that will clamp a signal to a different dc level. The network musthave a capacitor, a diode and a resistive element, but it can also employ an independent dcsupply to introduce an additional t = RC is large enough to ensure that the voltage across thecapacitor does not discharge significantly during the interval the diode is nonconducting.Throughout the analysis we will assume that for all practical the capacitor will fully charge ordischarge in five time constants.

General function

A clamping circuit (also known as a clamper) will bind the upper or lower extreme of a waveformto a fixed DC voltage level. These circuits are also known as DC voltage restorers. Clampers canbe constructed in both positive and negative polarities. When unbiased, clamping circuits will fixthe voltage lower limit (or upper limit, in the case of negative clampers) to 0 Volts. These circuitsclamp a peak of a waveform to a specific DC level compared with a capacitively coupled signalwhich swings about its average DC level (usually 0 V).

Clamp circuits are categorised by their operation; negative or positive and biased and unbiased.A positive clamp circuit outputs a purely positive waveform from an input signal; it offsets theinput signal so that all of the waveform is greater than 0 V. A negative clamp is the opposite ofthis - this clamp outputs a purely negative waveform from an input signal.

A bias voltage between the diode and ground offsets the output voltage by that amount.

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For example, an input signal of peak value 5 V (VIN = 5 V) is applied to a positive clamp with abias of 3 V (VBIAS = 3 V), the peak output voltage will be

VOUT = 2VIN + VBIAS

VOUT = 2 * 5 V + 3 V

VOUT = 13 V

Positive Clamper

The circuit for a positive clamper is shown in the figure. During the negative half cycle of the input

signal, the diode conducts and acts like a short circuit. The output voltage V0 = 0V. The capacitor

is charged to the peak value of input voltage Vm. and it behaves like a battery. During the positivehalf of the input signal, the diode does not conduct and acts as an open circuit. Hence the output

voltage Vo = Vm + Vm. This gives a positively clamped voltage.

Negative Clamper

During the positive half cycle the diode conducts and acts like a short circuit. The capacitor

charges to peak value of input voltage Vm. During this interval the output Vo which is taken

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across the short circuit will be zero. During the negative half cycle, the diode is open. The output

voltage can be found by applying KVL.

-Vm Vm Vo = 0

Vo = -2Vm

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2.4 Zener Diode

Schematic symbol and physical structure of Zener diode

A Zener Diode is a special kind of diode which permits current to flow in the forward direction as

normal, but will also allow it to flow in the reverse direction when the voltage is above a certain

value - the breakdown voltage known as the Zener voltage. Zener diode is specifically designed

to operate under reverse breakdown conditions. These diodes have a very accurate and specific

reverse breakdown voltage. The Zener voltage of a standard diode is high, but if a reverse

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current above that value is allowed to pass through it, the diode is permanently damaged. Zener

diodes are designed so that their zener voltage is much lower - for example just 2.4 Volts. When

a reverse current above the Zener voltage passes through a Zener diode, there is a controlled

breakdown which does not damage the diode. The voltage drop across the Zener diode is equal

to the Zener voltage of that diode no matter how high the reverse bias voltage is above the Zener

voltage.

Zener model can be employed for the ON and OFF state.

ON state - the output voltage is equal to the output voltage of the zener diode, Vz

OFF state - the output voltage is less than zener diode, Vz but greater than 0 V

A Zener diode transfers current forward and backward, while traditional diodes are only capable

of transferring current forward. Many different electronic systems use Zener diodes to control their

circuits voltage levels. While there are several ways of doing this, Zener diodes provide efficient

voltage regulation without damaging the diode itself.

How Zener Diodes Work

For the most part, a Zener diode acts like a regular diode as current moves in a forward direction

in its circuit. However, when a large enough voltage is applied to the diode in the reverse

direction, it travels against the current from the output to the input. Generally, this produces a

current build-up that eventually damages the diode. However, Zener diodes can limit the amount

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of current that passes through them so that the voltage going backward limits the voltage going

forward.

Applications

Zener diodes are used in a number of electronic devices but are exclusively used to control

voltage levels within an electrical circuit. They are especially used in power supplies where

alternating current is converted into direct current. These types of power supplies typically

transfer very large voltages and must limit the voltage before it is transferred to an electronic

device. These devices are often referred to as rectifiers and either a Zener diode or several

transistors can control them. However, in the case of one-chip regulators, transistors are too large

and a single Zener diode can perform the same function.

Advantages

Zener diodes are very small and can be integrated into virtually any electronic device, especially

small circuits that cannot accommodate other forms of voltage regulation technology. Zener

diodes are also relatively inexpensive and are compatible with most other systems, making them

the preferred voltage regulation device. Zener diodes can be used as a reference voltage where

it will provide a level for comparison against other voltages and as voltage regulator diode.

Disadvantages

Zener diodes cancel out voltages by applying even larger voltages in the reverse direction,

wasting electricity in the process. Additionally, Zener diodes have a relatively poor regulation ratio

and are generally not as good as transistors.

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I-V Characteristic of Zener Diode

Zener Diodes are used in the "REVERSE" bias mode, i.e. the anode connects to the negative

supply and from its I-V characteristics curve above, we can see that the Zener diode has a region

in its reverse bias characteristics of almost a constant voltage regardless of the current flowing

through the diode. This voltage across the diode (it's Zener Voltage, Vz) remains nearly constant

even with large changes in current through the diode caused by variations in the supply voltage

or load. This ability to control itself can be used to great effect to regulate or stabilise a voltage

source against supply or load variations. The diode will continue to regulate until the diode current

falls below the minimum Iz value in the reverse breakdown region.

2.5 The other type of Diodes

a) Light emitting diode (LED)

b) Photodiode

c) Laser Diode

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